An atomic force microscope (AFM), as well as a scanning force microscope (SFM) or a scanning tunneling microscope (STM), is an instrument in which a flexible cantilever of very small dimension is scanned across a surface. Conventional cantilevers are fabricated by micro machining techniques. Currently, there are two basically different ways to make cantilevers. The first uses thin films deposited onto a silicon substrate. The cantilever is then defined by lithography and etched into the thin film by wet or dry etching, followed by selective removal of the silicon underneath the cantilever. In the second fabrication method, the cantilever is micro machined out of a bulk silicon. This is done by etching through a window in a mask layer from the backside until the thickness equals twice the desired thickness of the cantilever. The cantilever is then defined and patterned by lithography on the front side and consecutively etched into the silicon until both etch fronts meet and the cantilever is released. Operating in reverse order is also possible by adding to the process a deposition step with a masking layer in order to protect the front side during backside thinning. The methods are described, for example, in Journal of Vacuum Science and Technology, Vol. B 9, No 2, March/April 1991, pp. 1353 to 1357.
The tip of a cantilever is the most crucial part. The resolution of a picture of a sample surface depends on the quality of the tip. That requires a well defined tip with known cone angle and sharp top apex.
Tips can also be either prepared as thin film tips using a silicon substrate or etched out of a silicon wafer by the following steps. Starting with a photolithographically patterned circular mask the silicon is etched away in the surroundings of the mask. Simultaneously, the mask is undercut until it is completely under etched. When the mask is released and the etching is stopped a silicon cone remains. An example for the microfabrication of a tip is disclosed in the article "Silicon cantilevers and tips for scanning force microscopy", J. Brugger et al., Sensor and Actuators A, Vol. 34, 1992, pp. 193-200. Large tips can be made by means of electroplating. To achieve this, a plating base is required to define the position where the tip is to be formed.
A disadvantage of these methods is that there remains residues of the etching. The usual fabrication of cantilevers with integrated tips needs at least three photolithographic steps and several dry and wet etching steps. The labour required for the fabrication is complex and expensive.
It is an object of the present invention to overcome the disadvantages of known approaches.
It is a further object of the present invention to provide a method for cost efficient fabrication of cantilevers with tip.
It is another object of the present invention to provide a simpler method for the fabrication of cantilevers with tip.